Designers Jessica Weigley and Kevin Hackett use organic materials whenever possible in their interior design practice at Siol Studios in San Francisco. Their design for a children's play space at this year's San Francisco Decorator Showcase highlighted recycled materials, sustainably harvested woods and a terrace garden with wind- and solar-powered irrigation features.

However, the area most attractive to visitors of all ages was their 36-square-foot secret room hidden behind a pivoting bookshelf. A window that looks onto the Presidio makes the room seem deceptively large from inside. Its floor, divided into two opposing 3-foot-wide curved plywood ramps, is swathed in the most durable, stain-resistant and organic material in Siol's repertoire: wool.

"It is an ancient material, and we really enjoy its authenticity," says Weigley, 32, a former teacher with the Peace Corps in Africa.

Made of fibrous tufts obtained from sheep, goats, camels and even rabbits, wool is elastic and can be woven or compacted (felted) into a fabric that is warm even when it absorbs a lot of moisture. It can hold air within its weave and thus has great insulating qualities, making it as effective for garments in the Highlands of Scotland, where it is damp, as it is for Berbers in the deserts of Africa. It also happens to be ideal for carpeting because it can take a lot of wear and tear and, in homes and offices, its flame resistance makes it superior to cotton or synthetic options. It does not off-gas toxic fumes. In some instances, when exposed to flame, it is even self-extinguishing.

For their fantasy room, where children conjure up a tree-climbing fantasy or read in the glow of LED strip lights, Weigley and Hackett picked a thick shag Berber weave wool carpet by Los Angeles-based Mat the Basics that "you can sink your fingers in," Weigley says. For a cocooning effect, it also covers one wall up to the ceiling. The organically dyed oatmeal-color rugs, sustainably produced in India with New Zealand wool, have the RugMark's GoodWeave certification, ensuring that no child labor was used in its manufacture.

"We designed this room for 5-year-olds," Weigley says. "But such a space takes us all back to our childhoods. It's great to touch."

At a glance

Expert opinion: "There are many more durable Ultrasuede and synthetic options to wool," Jessica Weigley says, but wool is softer and won't scruff knees. "The closet is a sanctuary and the material speaks to that," she says.

Pros: Wool comes in several natural shades of black, white, brown, silver and gray. It is durable and stain resistant.